Unguided Imaging with a SCT


Bruce Donzanti
 

I am guiding with an OAG on a C11" EdgeHD @ f/10 with a ZWO ASI6200mm Pro but finding guide stars all night long can sometimes be a challenge.  My primary mirror is locked down and focus is performed with the Optec Secondary Mirror FastFocus System.  So, mirror flop and shifting are non-issues.   Thus, I am curious if folks have successfully modeled and performed unguided imaging on a SCT.


Roland Christen
 

1) What mount do you have.

2) there is On-axis guiding also a possibility.

Rolando



-----Original Message-----
From: Bruce Donzanti <donza2735@...>
To: main@ap-gto.groups.io
Sent: Wed, Apr 20, 2022 1:51 pm
Subject: [ap-gto] Unguided Imaging with a SCT

I am guiding with an OAG on a C11" EdgeHD @ f/10 with a ZWO ASI6200mm Pro but finding guide stars all night long can sometimes be a challenge.  My primary mirror is locked down and focus is performed with the Optec Secondary Mirror FastFocus System.  So, mirror flop and shifting are non-issues.   Thus, I am curious if folks have successfully modeled and performed unguided imaging on a SCT.

--
Roland Christen
Astro-Physics


W Hilmo
 

I have imaged quite a bit with an 8" EdgeHD.

In my opinion, an OAG (or ONAG) is required with these scopes.  I did my testing on an AP1600 with Absolute Encoders and a tracking model through APCC.  With that configuration, I had pretty noticeable elongation of stars with 2 minute exposures on an ASI1600 at F/7.

The mirror "locks" on an EdgeHD are not locks at all.  The mechanism consists of two flexible rods that run from the back of the scope, to the back of the primary mirror carrier.  They are placed at 120 degrees in each direction from the focus knob.  When you tighten the two knobs on the scope, you are preventing the rods from moving forward and backward.  Since they are connected to the mirror carrier, this prevents the two contact points on the mirror carrier from moving forward and backward.  This should be reasonably effective for preventing mirror tilt, but only if you finish focus using the primary mirror focus knob with a counter-clockwise turn.  Finishing with a clockwise turn may allow for some tilt due to backlash on the threaded focus rod.

The bigger issue is that there needs to be sufficient "looseness" between the primary mirror carrier and the baffle tube, so that it can smoothly move to focus.  The mechanism that I described above cannot do anything to prevent lateral movement of the mirror in that case.

Back when I was just getting started with the scope, I noticed the problem.  I spent a considerable amount of time investigating and testing with the scope in various orientations.  I tested unguided (with the very capable mount and model described above), guided with a guide scope, and guided with an OAG.

Both the unguided and guide scope tests gave essentially identical results.  Interestingly, I got the exact same results with the mirror "locks" both loose and tight.  I suspect that this means that I have very little tilt, but some lateral movement.

When I tested with an OAG, I got perfectly round stars, regardless of the duration of the exposure.  I tested up to 30 minutes.

I have heard some people say that they are satisfied with the results that the get with a guide scope on an EdgeHD.  I don't know if that means that there is significant variability in the tolerances, or whether other folks are just less picky than I am (although the problem seems very obvious to me).

I would definitely recommend guiding with these OTA's, specifically using something that uses the same light path as the main camera.

I hope that this helps,
-Wade

On 4/20/22 11:51 AM, Bruce Donzanti wrote:

I am guiding with an OAG on a C11" EdgeHD @ f/10 with a ZWO ASI6200mm Pro but finding guide stars all night long can sometimes be a challenge.  My primary mirror is locked down and focus is performed with the Optec Secondary Mirror FastFocus System.  So, mirror flop and shifting are non-issues.   Thus, I am curious if folks have successfully modeled and performed unguided imaging on a SCT.


Bruce Donzanti
 

AP1100

I saw the ONAG by InnovationsForesight - not sure if there are others.  While they seem helpful in explaining what I need, it seems to use a particular focusing software that may not work well with N.I.N.A., the acquisition software I use.  Maybe I am wrong and need to investigate further.


Roland Christen
 

AP1100 with or without encoders?

Rolando



-----Original Message-----
From: Bruce Donzanti <donza2735@...>
To: main@ap-gto.groups.io
Sent: Wed, Apr 20, 2022 4:18 pm
Subject: Re: [ap-gto] Unguided Imaging with a SCT

AP1100

I saw the ONAG by InnovationsForesight - not sure if there are others.  While they seem helpful in explaining what I need, it seems to use a particular focusing software that may not work well with N.I.N.A., the acquisition software I use.  Maybe I am wrong and need to investigate further.

--
Roland Christen
Astro-Physics


Bruce Donzanti
 

Thanks, Wade.  I understand perfectly how the so-called "locks" (not really locking) work and have been using the Celestron OAG for several years.  It works well and I get excellent star shapes.  That is not the problem.  Now that I run through the entire night, there is the occasion when on meridian flip, there are no good guide stars for PHD2 as very few are present in the OAG FOV.  The Optec SMFFS is the best accessory I have ever bought for the Edge and the Optec locking knobs really tighten down the primary.  I've gone months with great success imaging throughout the night with the exception I mentioned above.  


Bruce Donzanti
 

Without in a permanent setup.


Cal Sandfort
 

The particular focusing software mentioned when using the InnovationsForesight's ONAG is not really required unless you want to have continuous focusing as you expose images. Otherwise, you can use your existing methods. The only difference will be that the guide stars will be more or less in and out of focus because of offsets from different filters but guiding algorithms typically handle this very well.


Worsel
 

Bruce

I have a 14.5" corrected Dall-Kirkham (f.l. 2470 mm) on a non-AE 1100 in a permanent observatory.  Typical setup is an image scale of 0.4 "/pxl!

With a 200 or more point APPM model, I can go unguided for up to 15 minutes at times.  It is likely related to the Dec of the target, but I can't quantify that theory.  I have not tried Dec arc modelling yet.

Attached is a comparison of three images with the above setup.  "Corrected tracking" = using the APPM derived model.

My conclusion is that, while unguided at this focal length is achievable, the real goal is good images.  Hence, I tinker with unguided, but I usually guide.  OAG and PHD2.

With a 910 mm focal length refractor (image scale 1.5), I image unguided all the time.  PHD2 is fine when needed, but it adds a layer of potential 'disruption.'  Its all a tradeoff!

Bryan


Bruce Donzanti
 

I just found out that InnovationsForesight is working with NINA on a native integration


Christopher Erickson
 

There is only one way I have found to be able to run unguided SCT's on any mount, and that is by adding a Hyperstar. At f2.2, exposures are rather short.
Floating/flopping mirrors cause much less problems with shorter exposures. The super-wide-field is great for many objects but not so great for others. For planets, I go with video modes and short exposures so guiding isn't very critical there either.

When I'm not using a Hyperstar or shooting planets, I always use an off-axis guider. I would like to play with an on-axis guider someday but I haven't yet made the investment.

Piggyback guiders on SCT's and most other compound scopes just leads to tears.

"Always take the high road. There's less traffic."

-Christopher Erickson
Observatory Engineer
Summit Kinetics
Waikoloa, Hawaii


On Wed, Apr 20, 2022 at 1:06 PM Bruce Donzanti <donza2735@...> wrote:
I just found out that InnovationsForesight is working with NINA on a native integration